1. Field of the Invention
The present invention relates to a photoelectric conversion element (which may be referred to as a "element" hereinafter), and in particular relates to an inexpensive photoelectric conversion element.
2. Discussion of the Related Art
Global warming due to combustion of fossil fuel and the growing energy needs associated with the population growth have become problems associated with extinction of the human race. It is needless to say that sunlight has developed the environment of the Earth and has been an energy source for all the lives including the human race from the earliest times until today. In recent years, sunlight is contemplated to be utilized as a clean energy source that is unlimited and produces no hazardous substances.
Among others, solar batteries which convert the light energy into the electrical energy, receive attention as a promising technology. Single crystal, polycrystal, or amorphous silicon, or compound semiconductors, such as CuInSe, GaAs, or CdS, are used as photovoltaic materials for the solar batteries. Since the solar batteries using these inorganic semiconductors exhibit relatively high energy conversion efficiency between 10% and 20%, they are widely used as power supplies for remote sites, or auxiliary power supplies for portable small electronic equipment.
From the viewpoint of the purpose of cutting back the fossil fuel consumption to prevent the environment of the Earth from becoming worse as described above, however, at the present time, it cannot be said that the solar batteries using the inorganic semiconductors are sufficiently effective. That's because these solar batteries using the inorganic semiconductor are produced in a plasma CVD or high temperature crystal growth process, so that much energy is required to produce their elements. In addition, they contain components, such as Cd, As, or Se, which may have a deleterious effect on the environment, so that there is a possibility that the discarded photoelectric conversion elements may result in an environmental destruction.
In addition, many solar batteries using organic materials which are intended to provide larger areas and lower prices have been proposed (for example, see Japanese Patent Laid-OpenNo. 53-131782 (1978), Japanese Patent Laid-Open No. 54-27387 (1979), Japanese Patent Laid-Open No. 56-35477 (1981), Japanese Patent Laid-Open No. 1-215070 (1989), Japanese Patent Laid-Open No. 4-10576 (1992), and Japanese Patent Laid-Open No. 6-85294 (1994) , those patents disclose organic solar cells having typically a first electrode having a light transparency, a second electrode and a single or a function-separated double layer organic photoelectric conversion element, but they are not yet in actual use because of their low conversion efficiency and low durability.
Under these circumstances, a photoelectric conversion element using semiconductor particulates sensitized by a coloring matter (hereinafter referred to as coloring matter sensitized photoelectric conversion element), a materials used for producing this element, and a method for producing this are disclosed in Nature (vol. 353, pp. 737-740, 1991), U.S. Pat. Nos. 4,927,721, 4,684,537, 5,350,644, 5,463,057, 5,525,440, 5,084,365, Japanese Patent Laid-Open No. 1-220380 (1989) that is Japanese equivalent of U.S. Pat. No. 4,927,721, and Japanese Patent Publication No. 8-15097 (1992) that is Japanese equivalent of U.S. Pat. No. 5,350,644. This coloring matter sensitized photoelectric conversion element is characterized in that a porous thin film of titanium dioxide spectrally sensitized with a ruthenium complex is used for a working electrode. Furthermore, since in this coloring matter sensitized photoelectric conversion element, inexpensive oxide semiconductors can be used without refining them to a high degree of purity, it is expected to provide a relatively inexpensive photoelectric conversion element. In addition, in this coloring matter sensitized photoelectric conversion element, the coloring matter used has a wide wavelength absorption range, and a high energy conversion efficiency of approximately 10% (AM 1.5) is attained.
On the other hand, it is desirable that a transparent electrode layer on the side of light-receiving face required in the structure of a common photoelectric conversion element has properties of low resistance and high transmissivity of visible rays, and a transparent electrode layer produced by forming a transparent conductive oxide film on a transparent insulating substrate, such as glass or plastic film, is presently used. However, the transparent conductive oxide film having such properties are produced in a sputtering or vacuum deposition process, so that its production requires a large-scale facility and much energy. Therefore, under the present circumstances, although the coloring matter sensitized photoelectric conversion element is relatively inexpensive, the cost for producing the transparent electrode layer in the photoelectric conversion element makes up the most part of its total production cost.
Also Japanese Patent Laid-Open No. 5-67797 (1993) discloses the same kind of coloring matter sensitized photoelectric conversion element having a light transparent electrode comprising an ITO film characterized in X-ray diffraction pattern. Japanese patent Laid-OpenNo. 6-275130 discloses an transparent conductive film formed on a substrate, the transparent conductive film having an oriented crystalline plane to the surface of the substrate and more than 80% transparency to visible light.
And, the conversion efficiency required for the photoelectric conversion element depends on the working conditions of the element, that is, load, power of radiated light, wavelength distribution, or size of the element. For example, the conversion efficiency required in the case where the element is installed outside of a house to convert sunlight into electric power available as an alternative to electric power obtained by thermal or atomic power generation is different from the conversion efficiency required in the case where it serves as a power supply for portable equipment to convert interior light into electric power.